Random numbers, which occur in digital form as random bit sequences, are frequently needed in security-relevant applications. It is necessary to generate and use random numbers in asymmetrical authentication methods for example. In particular for RFID tags with security functionality the corresponding random numbers are to be generated with especially low hardware outlay. It is desirable in such cases only to use digital logic circuits which can be implemented with little effort.
In the past random number generators have typically been employed by using of analog noise sources of which the signals are digitized. Hybrid analog/digital circuits are expensive to implement however.
A random number generator almost entirely featuring digital logic circuits is described in WO 2006/015624 A1. This international patent application proposes the use of random phase fluctuations from ring oscillators constructed from digital gates. In accordance with WO 2006/015624 A1 however only very specific ring oscillators, namely Fibonacci or Galois ring oscillators with particular properties, are suitable for use in a random number generator. A circuit constructed in this manner is based on the assumption that the ring oscillators may not have any fixed points. For Fibonacci or Galois ring oscillators in particular this condition can be represented by mathematical equations, which must be fulfilled according to WO 2006/015624 A1. For a corresponding implementation it is thus first necessary to test whether a ring oscillator has fixed points. It is thus disadvantageous that the choice of oscillators to be considered is very restricted.